1. Field of the Invention
This invention relates to a projection television apparatus including cathode ray tubes of three primary colors; red, green, blue, on the inner surface of which a multilayered interference filter is disposed.
2. Description of the Related Art
Because of a high vacuum within the envelope of the cathode ray tube, the larger its size becomes, the more influences due to a stress of an atmospheric pressure or a thermal stress caused in the heating/cooling steps when a cathode ray tube is manufactured increase thereby making it difficult to design the cathode ray tube. As a result, instead of the cathode ray tube, a projection television apparatus has been used as display apparatus for use in a television having a size of 45 inches or more.
As the projection type television apparatus, there are two basic types of the projection television. One is a projection television having a cathode ray tube which incorporates a refecting mirror and a lens for aberration correction use. The other is a projection television provided with a lens refraction at its exterior. Since advances have been made in the field of projection lens technique since and it has become possible to manufacture a projection lens having a large aperture and capable of obtaining a sufficient brightness at a low cost, the projection television of a refraction type is now extensively employed.
FIG. 3 of the accompanying drawings is a schematic plan view illustrating a structure of a conventional lens unit and a fragmentary enlarged plan view of a display window disposed inside of a cathode ray tube. At the inside of a cathode ray tube 1, there are provided a face plate pane 4, a fluorescent material layer 6 disposed on the innermost of the cathode ray tube 6 and a multilayered interference filter 5 inserted between the display face plate pane and the fluorescent material layer. When electron beams are incident on the fluorescent material layer 6, light having a predetermined wavelength is emitted from the fluorescent material layer 6 and radiated in a forward direstion out of the cathode ray tube passing through the multilayered interference filter 5 and the display face plate pane 4.
Further, a series of projection lenses 2 having the incident pupil at the center thereof are positioned in front of the cathode ray tube 1, and an image is produced on a screen (not shown) by reflecting light emitted from the cathode ray tube 1.
Still, the multilayered interference filter 5, disposed on the inner surface of the cathode ray tube 1, is composed of alternately superimposed high and low refractive index materials, and the distribution of luminous intensity emitted from the fluorescent material layer 6 is varied by interference of a beam of light. In the fragmentary enlarged plan view, there are illustrated the distribution of luminous intensity 7 produced by the cathode ray tube having the multilayered interference filter 5 and the distribution of luminous intensity 8 produced by the cathode ray tube having no multilayered interference filter 5. The lesser becomes an angle .theta. which a light-emitting phosphor dot of the fluorescent material layer 6 forms with respect to the incident pupil 3 positioned at the center of a display window 12. In other words, the closer the light-emitting phosphor dot is located to the center of the display window 12, the more the luminous intensity increases. In addition, it should be understood that the more the angel .theta. increases, namely, the farther the light-emitting phosphor dot is located from the center of the display window 12, the smaller the luminous intensity of the cathode ray tube having the multilayered interference filter 5 becomes, compared to that of the cathode ray tube having no multilayered interference filter 5.
In a structure, a conventional projection television apparatus comprises a combination of cathode ray tubes for three primary colors, i.e., red, green, blue. The green and blue cathode ray tubes have a multilayered interference filter 5 disposed on the inner surface of the cathode ray tube 1 in order to improve the brightness, whereas the red cathode ray tube is not equipped with the multilayered interference filter because it has an allowance for further increase of beam currents necessary for enhancing the brightness. This thereby reduces the cost of the apparatus.
However, with this conventional structure in which only the red cathode ray tube has no multilayered interference filter, there is a disadvantage in that the display color becomes red-emphasized at the peripheral portion of a screen due to discrepancies of the luminous intensity as shown in the fragmentary enlarged plan view of FIG. 3.
SUMMARY OF THE INVENTION
Accordingly, this invention is made to solve the above-mentioned drawbacks of the prior art. An object of the present invention is to provide a projection television apparatus capable of producing uniform color almost on the entire screen without increasing the production cost.
To this end, a projection television apparatus, in accordance with the present invention, comprises a red cathode ray tube having no multilayered interference filter and a series of projection lenses with a diaphragm for both refracting light emitted from the fluorescent material layer disposed in front of the red cathode ray tube and decreasing the luminous intensity at the peripheral portion of the screen relative to that at the screen center.
The series of projection lenses with the diaphragm include a series of projection lenses capable of reflecting light emitted from the fluorescent material layer of the red cathode ray tube and the ring-shaped diaphragm arranged to form a concentric circle to the central axis of the series of projection lenses.
In practice, the series of lenses with the diaphragm disposed in front of the red cathode ray tube decreases the luminous intensity around the peripheral portion of the screen when light emitted from the red cathode ray tube is displayed on the screen. This thereby obtains a luminous intensity distribution nearly equal to that of light radiated from the green and blue cathode ray tubes having the multilayered interference filter. Accordingly, the red-emphasized display color is eliminated from the periphery of the screen, and it becomes possible to obtain an image displayed with uniform color.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.